Method and structure for automated inert gas charging in a reticle stocker

ABSTRACT

At least a first reticle is stored in a housing of a stocker. A first gas is delivered to the housing. At least one reticle pod having an additional reticle is delivered into a enclosure within the housing of the stocker. A second gas different from the first gas is delivered to the enclosure. The reticle pod is automatically retrieved from the enclosure. The delivery and retrieval of the reticle pod and delivery of the first gas and the second gas are automatically controlled.

This application claims the benefit of U.S. Provisional Application No. 60/968,361, filed Aug. 28, 2007.

FIELD OF THE INVENTION

The present invention relates to control systems generally, and more specifically to a control method and system for stockers in an automated material handling system (AMHS).

BACKGROUND

AMHS's have been used extensively in the semiconductor fabrication field. The typical system includes a plurality of bays (rows) of storage areas. Each bay has a stocker, which includes bins for holding a plurality of containers, such as standard mechanical interface (SMIF) containers for loading 200 mm (8 inch) wafers, or front opening unified pods (FOUPs), which may be used to load 300 mm (12 inch) wafers. The stocker holds the SMIFs or FOUPs in preparation for transporting a SMIF or FOUP to the loadport of a processing tool. In technologies using larger critical dimensions (CD) and 200 mm wafers, a reticle stocker stored and delivered reticles for use in photolithographic processes. An overhead hoist transport (OHT) associated with each bay transports the SMIF or FOUP with wafers, or the pod with a reticle, from the respective stocker to a loadport for processing in one of the tools (fabrication process machines).

In conventional technologies using reticles with larger CDs, it was common to cover each reticle with a pellicle. The pellicle was a thin film of polymer secured to the reticle to prevent dirt from collecting directly on the reticle. The pellicle kept any foreign particles outside of the focal plane of the photolithographic tool used for patterning the wafer. For higher technology fabrication processes (90 nm, 65 nm and technologies with smaller critical dimensions), the pellicle material is not transparent to the extreme ultra-violet (EUV) radiation used. Because of this, higher technology reticles are often provided without a pellicle. Without the protection of the pellicle, other means are used to protect the reticle from contamination and precipitation of foreign substances.

It is known that charging (exposing) the reticle in a nitrogen (N₂) atmosphere immediately before and immediately after the reticle is used for patterning a wafer will purge the reticle of impurities. The impurities diffuse out of the reticle material into the purging gas.

Thus, as shown in FIG. 1, the higher technology (smaller CD) reticles 201 were stored in a separate cabinet 202 and manually delivered by a worker 203 to an N₂ charging tool 204 before and after use in the fabrication tool 205. A semiconductor fabrication facility (“fab”) may have many low volume density N₂ cabinets 202 that consume significant space. These cabinets 202 were hard to manage because they were in dispersed locations. The handling procedure for reticle pod N₂ charging by the operator was also complicated. This impacted the operation efficiency of the tool 205. In some instances, delay occurred when a lot 206 of wafers was ready for processing by the tool 205, but the N₂ purged reticle 201 was not yet available at the tool. Errors in transferring the reticles 201 to and from the N₂ cabinet 202 and failing to properly perform the N₂ purge operation caused high reticle repairing rates and lots were often scrapped due to precipitation of materials on reticles 201 that were used to process those lots.

SUMMARY

A method for storing and delivering a reticle comprising storing at least a first reticle in a housing of a stocker. A first gas is delivered to the housing. At least one reticle pod having an additional reticle is automatically delivered into an enclosure having a separate ambient environment within the housing of the stocker. A second gas different from the first gas is delivered to the enclosure. The reticle pod is automatically retrieved from the enclosure. Delivery and retrieval of the reticle pod and delivery of the first gas and the second gas are automatically controlled.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a conventional process for retrieving a reticle pod from a storage cabinet, performing an N₂ charging operation, and delivering the reticle to a tool.

FIG. 2 is a block diagram of an exemplary embodiment of an automated reticle stocker.

FIG. 3 is another block diagram of the system of FIG. 2, showing the control components.

FIG. 4 is a flow chart of operation of the control elements shown in FIG. 3.

FIG. 5A is a partial cutaway isometric view of the stocker shown in FIGS. 2 and 3.

FIG. 5B is an enlarged detail of the enclosure (purge nest) of FIGS. 2 and 3.

FIG. 6 is a flow chart of an exemplary method of automatically controlling and operating the reticle stocker.

FIG. 7 is a flow chart showing a method of assigning priority for purge operations performed in the reticle stocker.

DETAILED DESCRIPTION

This description of the exemplary embodiments is intended to be read in connection with the accompanying drawings, which are to be considered part of the entire written description. Terms concerning attachments, coupling and the like, such as “connected” and “interconnected,” refer to a relationship wherein structures are secured or attached to one another either directly or indirectly through intervening structures, as well as both movable or rigid attachments or relationships, unless expressly described otherwise.

Because the availability of reticles to process wafers at the time the equipment is ready to perform the processing impacts on the overall production rate, it is desirable to operate the AMHS in a manner that supplies reticles quickly as soon as they are needed.

A solution is described herein suitable for storage and haze prevention of advanced reticles, such as those used for 300 mm wafers, 450 mm wafers, and the like. The reticle stocker provides high reticle POD storage capacity for a phase shifting mask (PSM) that is required to be preserved in a specific environment by charging the reticles with an inert gas such N₂ or purified clean dry air (XCDA).

FIG. 1 shows the conventional process for charging reticles. Reticles 201 were stored in an N₂ cabinet 202. Then, a worker 203 would remove a reticle 201 and place it in an N₂ charge tool 204, which would charge the reticle. The reticle 201 was than moved to a scanner tool 205. Often this sequence would cause delays for waiting lots 206.

As shown in FIG. 2, instead of manually handling the reticle pods 2, the system 4 offers the function to forecast incoming work-in-progress (WIP) and automatically charge the reticle 8 with inert gas (e.g., N₂) within the reticle pod 2 both before and after using the reticle 8 for a photolithographic process.

One example is described below in which a reticle 8 may be stored within an automated stocker 10 in a first gas such as clean dry air (CDA) or purified CDA (XCDA), and the reticle 8, within its pod 2, can be automatically delivered to a purge nest 12 (an enclosure, within the housing 14 of the stocker 10, having a separate ambient environment from the rest of the stocker housing 14). The purge nest 12 has an N₂ delivery mechanism 16 (shown in FIG. 5A) for performing the charging operation to perform a nitrogen purge of contaminants from the reticle 8. One of ordinary skill in the art understands that a variety of first gases (other than CDA) may be used throughout the stocker 10, and a variety of second gases (other than N₂) may be used within the purge nest 12, for handling reticles 8 with different storage and processing requirements.

FIGS. 2, 3 and 5A show an exemplary reticle stocker 10 for storing semiconductor fabrication reticles 8. The stocker 10 includes a stocker housing 14. At least one receptacle 15 is provided within the stocker housing 14 for storing at least one reticle 8. Preferably a plurality of receptacles are configured in the housing 14 for containing reticles 8, or for containing reticle pods 2 that in turn contain reticles 8.

As best seen in FIG. 5A, the reticle stocker 10 includes a means for delivering a first gas to the at least one receptacle 2. In some embodiments, the first gas may be CDA or XCDA. The means for delivering the first gas may include a CDA (or XCDA) supply conduit 18. In some embodiments, the first gas is provided continuously into the housing 14, so that no control valves are needed within or near the housing 14 for the first gas. In other embodiments, a control valve 20 one-way may be provided in the stocker 10 for the first gas. The inlet gas should be passed through a chemical filter 22, and should be a downward flow, so that any foreign particles dislodged by the first gas fall down to the bottom of the stocker 14 for removal, and are not recirculated within the stocker 14. For example, the first gas inlet 24 may be placed at or near a top of the housing 14, and a gas outlet 26 may be near the bottom of the housing 14. Pressure in the stocker 14 is desired to be higher than outside ambient by at least 10 mm H₂O. The charging connectors 28 are preferably particle proof or at least not susceptible to damage from expected particle impact.

In alternative embodiments, where the stocker 10 is placed in a CDA (or XCDA) clean room environment, the housing 14 of the stocker 10 may include one or more openings to permit ingress and egress of the ambient CDA (or XCDA) to deliver the first gas.

In some embodiments, the stocker 10 has a sealable enclosure 12 within the stocker housing 14 for receiving at least one reticle pod 2 having an additional reticle 42 therein. For example, the sealable enclosure 12 may be a purge nest with closable valves.

In some embodiments, the purge nest 12, as shown in FIG. 5B, may include a plurality of vertically adjustable stages 502 for holding pods 2 of various sizes. For example, by raising the stage 502 of layer 2 by the distance 514, the inlet port and outlet port of pod 2 is brought into engagement with the charging port 506 and exhaust port 507 of the purge nest 12 for performing a purge operation. Raising and/or lowering the stages 502 of each layer also increase the distance 512 between layers to ensure adequate clearance during positioning, and good gas circulation during the purge operation. Each stage 502 may be supported by guide cylinders 510. A robot arm 508 may be provided to handle and position the pod 2 within the purge nest 12. Rotary clamp cylinders 504 may be provided to tightly close the pod 2 to ensure proper sealing.

In some embodiments, the purge nest 12 provides a charging port 506 designed to fit properly with the inlet port (not shown) of the reticle pod 2. The reticle pod 2 is connected to the charging port 506 and the purging gas (e.g., N₂) is delivered to the reticle pod 2. The charging port 506 may have a check valve (not shown) to prevent egress of gas from the reticle pod 2 back into the N₂ supply. The reticle pod 2 also has an outlet port 36, from which the N₂ can escape into the purge nest 12 by way of the exhaust port 507 of the purge nest 12. The purge nest 12 may further include another one-way check valve 38 for its output port 36, to prevent the ambient first gas (CDA or XCDA) in the housing 14 of the stocker 10 from entering the purge nest 12.

The stocker 10 has a means for delivering a second gas (such as N₂, XCDA or an inert gas) to the enclosure (purge nest) 12. For example, the means for delivering the second gas may include a pipe 44 that delivers the second gas, under control of a flow control valve (not shown) that is controlled by an N₂ controller 46.

As best seen in FIG. 5A, the stocker 10 has at least one robot 48 for delivering a reticle 8 to the at least one receptacle 15 and for delivering the reticle pod 2 to the purge nest enclosure 12. The at least one robot 48 also retrieves the reticle 8 from the at least one receptacle 15 and retrieves the reticle pod 2 from the purge nest enclosure 12. In some embodiments a single robot handles all of these functions. In other embodiments a plurality of robots are provided, and the robot tasks may be divided among the plurality of robots, either by function or by the location of the reticles 8 within the stocker 10.

The stocker further includes at least one controller 50 for controlling operation of the robot 48 and for controlling the first gas delivering means and the second gas delivering means. FIG. 5A shows two separate controllers 46, 50 for the stocker 10 and for the N₂ purge nest 12. Additional control functions may be performed by the same or additional processors. For example, control and monitoring functions related to the work-in-progress (WIP) 6 in the fab may be performed by the manufacturing execution system (MES 52), and control and monitoring functions related to disposition and movement of individual pods may be made by the material control system (MCS 54). The various control functions may be physically distributed or collocated, or contained within a single programmed processor, or implemented in special purpose hardware (application specific integrated circuitry, or ASIC).

Various functions are performed within the controller 50. The various means within the controller 50 described below may correspond to software modules in a one-to-one relationship, a one-to-many relationship, or a many-to-one relationship.

In some embodiments, the at least one controller 50 includes means for scheduling the N₂ charge operation by scheduling the at least one robot 48 and the second gas delivering means for exposing the additional reticle 42 to the second gas (N₂) within the purge nest enclosure 12. Preferably, the scheduling is based on at least one rule that determines a duration and/or a schedule of the exposing (charging) based on a fabrication process to be performed using the reticle 8.

For example, the at least one controller 50 may includes means for controlling the robot 48 to place the pod 2 containing a given reticle 8 in the purge nest 12, and controlling the second gas delivering means to deliver N₂ to expose the additional reticle 42 to the second gas (N₂) in the purge nest enclosure 12 for a first predetermined period of time. The first predetermined period may be 10 minutes, 15 minutes, or whatever period is empirically determined to be suitable for purging contaminants from a particular type of reticle used in a particular process.

In some embodiments, the controller 50 includes means for scheduling the at least one robot 48 and the second gas delivering means for exposing the additional reticle 42 to the second gas within the purge nest enclosure 12 immediately before the reticle 8 is to be delivered for use. Then, after the reticle 8 is used in the semiconductor photolithographic process, the operator puts the reticle pod 2 into the stocker 10 after the mask 56 is used, causing a system triggered N₂ purge operation. Thus, the reticle 8 is N₂ purged shortly before and shortly after it is used in the photolithographic process. By performing the N₂ purge operation shortly after the reticle 8 is used, the reticle 8 is preserved, and the reticle 8 is also ready if it is needed for another photolithographic process soon thereafter.

In some embodiments, the at least one controller 50 includes means for scheduling the at least one robot 48 and the second gas delivering means for exposing the additional reticle 42 to the second gas within the purge nest enclosure 12 for a first predetermined period of time (e.g., 10 minutes or 15 minutes), if a second predetermined period of time (e.g., one hour or two hours) will have elapsed between a time when the additional reticle 42 was most recently exposed to the second gas and a time when the additional reticle 42 is to be used to process a wafer (not shown). That is, in some embodiments, after an N₂ purge operation is performed on the reticle 8, the reticle 8 is ready for use within a second predetermined period, such as two hours, after which reticle 8 is no longer ready for use and must again be subjected to an N₂ purge operation before it is used. When the controller 50 selects a reticle 8 for use with the forecasted WIP 6, the controller also determines whether the reticle 8 has “timed out” since it was last purged, or whether it will time out before it is scheduled to be used. In either case, the controller 50 schedules another N₂ charge operation before the reticle 8 is to be used. By scheduling an N₂ purge operation when the second predetermined period expires, the system 4 maintains a set of one or more reticles that are ready for immediate use when needed.

These advance purge operations may eliminate production delays that frequently occurred when the N₂ purging was performed manually at the last minute before the reticles were to be used. Also, by performing the purge operation in advance of when the operation is required, it may be possible to avoid queueing delay due to waiting for the availability of the purge nest 12, in the case where many reticles are needed within a short period of time.

In some embodiments, the at least one controller 50 includes means for determining when the additional reticle 42 is to be removed from the stocker 10 for use in a semiconductor fabrication process. Means are provided for scheduling the robot 48 and the second gas delivering means to expose the additional reticle 42 to the second gas for the first predetermined period of time, such that the N₂ purge operation (exposing) is completed before the time when it is desired to remove the additional reticle 42 from the stocker 10. In order to perform this function, the WIP 6 forecast by the MES 52 is used to predict when a given lot of wafers will progress to the point in the fabrication process when each reticle is needed, and the reticle N₂ purge operations can be scheduled so that the operations are completed before each reticle is to be used.

In some embodiments, the controller 50 includes means for forecasting a type of a wafer to be processed and forecasting when that wafer is to be processed. Also, means are provided for selecting the additional reticle 42 to be used for processing that wafer.

In some embodiments, the controller 50 is capable of operating in either a first mode in which the controller causes the second gas delivering means to deliver the second gas to the purge nest enclosure 12 during one or more discrete periods according to a schedule or a second mode in which the controller 50 causes the second gas delivering means to deliver the second gas to the purge nest enclosure continuously. Thus, if the duty cycle is high, it may be desirable to operate the purge nest 12 continuously in the second mode.

The exemplary stocker 10 provides a high volume density storage device in which a reticle POD 2 will be protected by inert gas (e.g., N₂), and performs pod charging by a sequence designed by users. The reticle stocker 10 not only saves fab space, but also improves operation productivity of scanner and people. The reticle stocker 10 improves operation efficiency by N₂ charge in the reticle pod 2 in advance of when the reticle 8 is needed for processing the WIP 6, to be ready before it needed. For example in a prior system of the type shown in FIG. 1, there were 36 sets/cabinets with high area density (about 176 cm2/Reticle), dispersed at various locations. A system as shown in FIG. 2 provides efficient area density with a footprint of about 60 cm2/Reticle, and the reticle stockers 10 may be concentrated.

The reticle stocker 10 may be implemented in new equipment. Alternatively, existing stockers may also be modified to perform the N₂ purge operation. For example, an existing stocker may be modified to include the purge nest, the means for delivering the second gas and the controller. For example, an N₂ piping system and flow design can maintain positive gas pressure within stocker. A variety of embedded N₂ nest designs can be configured, depending on the size of the pod and the number of pods to be charged simultaneously at any given time. The coming WIP forecast and scheduling algorithm can be implemented as a software modification in a system having a controller with computer program code.

In some embodiments, as shown in FIG. 3, an agent based operation (ABO) function may be used for implementing a prioritization scheme. In some embodiments, the prioritization is used in combination with the WIP forecast to schedule reticles to be processed on a priority basis.

FIG. 4 is a flow chart, showing one example of steps involved in the method of using the reticle stocker 10. The flow chart also notes where steps occur. Specifically, the chart notes the operation, steps performed at the STC 65, steps performed at the MCS 54/MES 52, actions taken at the real time dispatcher (RTD) 60, steps occurring under agent based operation, and actions carried out at TAP 62 (TAP is tool application program, whose function is to translate TCS command to the protocol compatible with Tools.). The flow chart indicates that at step 68 the POD 2 is put on the PGV inport 64, the POD ID is read at step 70, which is reported at steps 72, 74 to the MCS 54 and MES 52, in turn the POD ID, reticle ID, and STK ID are reported to TCS 60. The information is reported to check if the POD type was right or not, and to check whether the reticle needs to be charged or not. Next, verification of the ID at decision step 78 and verification of reticle purging at decision step 80 are performed. After verification step 78, macro and micro commands 82-88 are sent to dispatch the reticle to the shelf at step 90 and the POD location and status are updated 91. Also following the verification of IDs at step 78, a RTD priority 92 is determined at step 95 in the STC 65. Further, if the nest 12 is busy at step 93, the mask is sent at step 94 to the purge nest 12, which causes macro and micro commands to be issued at steps 96-104. The personal guide vehicle (PGV) port 64 goes 101 to the N2 port 101 having the ID number “port ID”. The PGV port 64 is the input/output port of the reticle stocker that can be operated manually. The commands execute a purge and update the POD location and status at step 106. The system also may issue an alarm 73 if a read fails at step 71.

An exemplary ABO 66 system considers several factors to setup priority and orders the MES 52/MCS 54 to execute reticle N₂ purge operations. These factors may include: (a) arrival time of coming WIP, (b) whether the Mask was just used or not (masks are purged as soon as possible after use), (c) the Mask Technology (e.g., N90, N65, etc.), and (d) the N₂ purge expired time (e.g., 2 hours), such that the N₂ purge is repeated if the reticle is not used within the second predetermined period after the most recent purge operation.

The priority system can be used in a variety of ways. For example, all N₂ purging operations for reticles that are forecast to be used within a given period (e.g., one hour) may be assigned high priority, and any reticle that is not forecast to be used within that given period can be assigned a lower priority. A reticle that was purged more than two hours ago, and requires another N₂ charge before use, may be assigned to a low priority if it is not forecast to be used within one hour. This prevents any N₂ purge operation that is performed in advance of the time the corresponding reticle is needed from causing delay in the delivery of a reticle that is to be used sooner.

Some embodiments of the method include automatically delivering the reticle pod 2 to the purge nest enclosure 12 and exposing the additional reticle 42 to the second gas in the enclosure 12 for a first predetermined period of time (e.g., 15 minutes). This occurs if a second predetermined period of time (e.g., 2 hours) will have elapsed between a time when the additional reticle 42 was most recently exposed to the second gas and a time when the additional reticle 42 is scheduled to be used to process a wafer. In this situation, the additional reticle 42 may be exposed to the second gas in the purge nest enclosure 12 before the expiration of the second predetermined period of time since the last purge, if the time between the current purge operation and the expected time of use of the reticle is less than the second predetermined period. This may eliminate queueing delays that could occur if the system were to always wait at least the second predetermined period between successive purge operations.

In some embodiments, the method further includes determining when the additional reticle 42 is to be removed from the stocker 10 for use in a semiconductor fabrication process. Exposure of the additional reticle 42 to the second gas in the purge nest enclosure 12 is scheduled for a first predetermined period of time. The scheduling is performed such that the exposing is completed before the additional reticle 42 is to be removed from the stocker 10.

FIG. 6 is a flow chart of an exemplary method of operating the stocker 10.

Step 600 includes storing at least a first reticle 8 in a housing 14 of the stocker 10. Preferably, a large number of reticles are stored in the housing 14.

At step 602, a first gas (e.g., CDA) is delivered to the housing 14.

Step 604 includes forecasting a type of a wafer to be processed and when that wafer is to be processed. The forecasting is used for determining when the additional reticle is to be removed from the stocker for use in a semiconductor fabrication process The forecasting may occur at regularly timed intervals (e.g., every one minute, or every five minutes), or the forecasting may occur continually. When the processing of a wafer is forecasted, the additional reticle 42 to be used for processing that wafer is selected.

At step 606, a determination is made whether the selected reticle is to be used at a time that is more than a second predetermined period (e.g., 2 hours) since that reticle was most recently exposed to the purging gas. That is, the interval between the last purge and the forecasted time of use is compared to the second predetermined period. If the time of use will be more than the second predetermined period after the last purge operation for that reticle, step 608 is performed. If the time will be less than or equal to the second predetermined period, then step 616 is performed.

At step 608, automatic delivery of the reticle pod 2 to the purge nest enclosure 12 and automatic retrieval of the pod from the purge nest are scheduled.

At step 610, at least one reticle pod 2 having an additional reticle 42 is automatically delivered into the purge nest enclosure 12 within the housing 14 of the stocker 10.

At step 612, the delivery and retrieval of the reticle pod 2 and delivery of the first gas and the second gas are controlled automatically.

At step 614, the second gas (e.g., N₂) different from the first gas is delivered to the purge nest enclosure 12 for the first predetermined period.

At step 616, the reticle pod 2 is automatically retrieved from the purge nest enclosure 12.

At step 618, the photolithographic step is performed using the reticle.

Some embodiments include exposing the additional reticle 42 to the second gas for the first predetermined period of time after the additional reticle 42 is used to process a wafer but before the additional reticle 42 is stored in the at least one receptacle 15 of the stocker 10.

In embodiments where a second purge is performed after each use, at step 620, the pod with the reticle therein is returned to the enclosure 12 within the reticle stocker 10.

At step 622, the reticle is again purged of contaminants within the enclosure 12.

At step 624, the reticle is returned to one of the storage compartments within the reticle stocker 12, until either the reticle times out (after the second predetermined period elapses), or until the reticle is again used for a photolithographic process.

Some embodiments include assigning a respective priority to each operation of exposing a respective additional reticle to the second gas. The scheduling step schedules for each exposing operation are based on the respective priority of that respective exposure.

FIG. 7 is a flow chart of an exemplary method of assigning the priorities for the purging operations. The example of FIG. 7 can be used in a system having two priority levels (high and low) or three priority levels (high, medium and low).

At step 700, the respective priority of charging (exposing) a given additional reticle is based on at least one factor from the group consisting of:

(a) an amount of time until a wafer is to be processed using the given additional reticle; (b) a forecast of wafers to be processed within a predetermined period of time; and (c) an amount of time since the given additional reticle was most recently exposed to the second gas.

At step 702, a determination is made whether a given reticle to be purged will be used in a photolithographic operation within a short period of time. For example, a third predetermined period of time—e.g., ten minutes from the time step 702 is performed—may be used as the threshold for this determination. If the reticle is scheduled to be used within the short (third predetermined) period of time, then step 703 is performed. If the scheduled time of use will not occur for more than the third predetermined period, then step 704 is performed.

At step 703, the reticle to be used within a short period of time is assigned the high priority.

An example in which a reticle is to be used in a short period of time is a case in which the reticle is to be used 2 hours and 10 minutes after the most recent purge, the purge operation requires 15 minutes, and the reticle cannot be used if it was last purged more than two hours ago. In such a case, the reticle must be scheduled to begin an additional purge operation before the two hour period since its last purge expires. Another example in which a reticle may be used within a short period of time after step 702 is performed is a situation in which an operator has manually changed the queue of photolithographic processing operations to be performed.

At step 704, a reticle that is not going to be used within the third predetermined (short) period is prioritized. If the reticle was just recently used in a photolithographic operation, then step 705 is performed. If the reticle was not recently used in a photolithographic operation, then step 706 is performed.

At step 705, the reticle may be assigned a high or medium priority, depending on the number of priority levels used in the system. In a two-level (high-low) system, the recently used reticle may be assigned high priority, along with reticles that will be used imminently. In a three-level (high-medium-low) system, the recently used reticle may be assigned high or medium priority. Assigning the recently used reticle a high priority allows the reticles that will be imminently used for photolithographic operations and the reticles that were recently used to be purged first, before any reticles that were previously purged after the last use.

Alternatively, assigning the recently used reticle a medium priority allows the reticles that will be imminently used for photolithographic operations to be purged first, to avoid production delays, and also allows recently used reticles to be purged before the reticles that were previously purged since their last use, but have timed out since the last purge.

In some embodiments, steps 706 and 707 may be skipped, and any reticle that will not be used imminently, and was not recently used is assigned low priority at step 708.

In other embodiments, at step 706, an optional third prioritization may be made, based on the time since the last purge operation. If step 706 is performed, and the time since the last purge operation exceeds the threshold second predetermined period, step 707 is performed. If the time since the last purge is less than the second predetermined period, step 708 is performed.

At step 707, in a two-level system, a reticle that has “timed out” (time since the last purge is greater than the second predetermined period) is assigned the low priority. In some three-level embodiments, a reticle that has “timed out” may be assigned medium priority, so that reticles which have timed out are purged before reticles that have not timed out. In other three-level embodiments, a reticle that has “timed out” may be assigned low priority, so that reticles that will be used imminently or were recently used are purged before any other reticles.

At step 708, any reticle that will not be used imminently, was not recently used, and has not “timed out” is assigned the low priority.

At step 710, each purge operation is scheduled based on the relative priority of the queue of purge operations to be performed.

Although two and three level priority schemes are described above, additional one or more level(s) may be used. For example, a four level system may assign priorities as follows: reticles to be used imminently are highest priority; recently used reticles are assigned second highest priority; reticles that have timed out since the last purge are assigned third highest priority, and reticles that have not timed out since the last purge are assigned the lowest priority.

Many variations are contemplated, for example, some embodiments include operating the controller 50 in either a first mode in which the controller 50 causes the second gas delivering means to deliver the second gas to the purge nest enclosure 12 during one or more discrete periods according to a schedule or a second mode in which the controller 50 causes the second gas delivering means to deliver the second gas to the purge nest enclosure 12 continuously. In some embodiments, the first gas is clean dry air, and the second gas is nitrogen or an inert gas.

The exemplary reticle stocker 10 creates a high volume density PSM reticle stocker to fulfill the requirement of storage, specific environment and charging in earlier stage to avoid delays when the wafer and tool are ready for processing, but the reticle 8 is not ready because it is being charged.

The exemplary reticle stocker 10 will make sure the reticle 8 can be kept in inert gas environment and required charging before and after the photolithographic wafer patterning process, without increasing fab space requirements. This stocker 10 can provide a high quality environment and handling of MASKs to avoid wafers being scrapped due to the reticle precipitation phenomenon and MASK repairing rate.

The stocker has potential to provide cost savings, reduce scanner idle time, and eliminate any need for a separate N₂ cabinet or N₂ Charging tool.

The functions of the various elements shown in the figures, including functional blocks labeled as “processors” or “controllers” may be provided through the use of dedicated hardware as well as hardware capable of executing software in association with appropriate software. When performed by a processor, the functions may be provided by a single dedicated processor, by a single shared processor, or by a plurality of individual processors, some of which may be shared. Moreover, explicit use of the term “processor” or “controller” should not be construed to refer exclusively to hardware capable of executing software, and may implicitly include, without limitation, digital signal processor (DSP) application specific integrated circuit (ASIC) hardware, read-only memory (ROM) for storing software, random access memory (RAM), and non-volatile storage. Other hardware, conventional and/or custom, may also be included.

The exemplary embodiments of present invention may be embodied in the form of computer-implemented processes and apparatus for practicing those processes. Other embodiments are in the form of computer program code embodied in tangible computer readable storage media, such as random access memory (RAM), floppy diskettes, read only memories (ROMs), CD-ROMs, DVD-ROMs, hard drives, flash drives, or any other computer-readable storage medium, wherein, when the computer program code is loaded into and executed by a computer, the computer becomes an apparatus for practicing the invention. Other embodiments are in the form of a computer having computer program code, loaded into and/or executed therein, wherein the computer becomes an apparatus for practicing the invention. When implemented on a general-purpose processor, the computer program code segments configure the processor to create specific logic circuits.

Although the invention has been described in terms of exemplary embodiments, it is not limited thereto. Rather, the invention should be construed broadly, to include other variants and embodiments of the invention, which may be made by those skilled in the art without departing from the scope and range of equivalents of the invention. 

What is claimed is:
 1. A method for storing and delivering a reticle comprising the steps of: (a) storing at least a first reticle in a housing of a stocker; (b) delivering a first gas to the housing; (c) automatically determining whether a predetermined time out period will have elapsed between a first time when an additional reticle was most recently exposed to a second gas different from the first gas and a second time when the additional reticle is scheduled to be used to process a wafer, wherein the timeout period is an amount of time after which the first reticle must be exposed to the second gas before using the first reticle to process the wafer; (d) automatically delivering at least one reticle pod having the additional reticle into an enclosure having a separate ambient environment within the housing of the stocker before the timeout period elapses, if step (c) determines that the time out period will have elapsed between the first and second times; (e) delivering the second gas to the enclosure; and (f) automatically retrieving the reticle pod from the enclosure.
 2. The method of claim 1 further comprising: exposing the additional reticle to the second gas in the enclosure for a first predetermined period of time if the time out period will have elapsed between the first time and the second time.
 3. The method of claim 2 further comprising: determining when the additional reticle is to be removed from the stocker for use in a semiconductor fabrication process; scheduling automatic delivery of the reticle pod to the enclosure; and exposing the additional reticle to the second gas in the enclosure for a first predetermined period of time, the scheduling being performed such that the exposing is completed before the additional reticle is to be removed from the stocker.
 4. The method of claim 3 further comprising: forecasting a type of a wafer to be processed and when that wafer is to be processed; and selecting the additional reticle to be used for processing that wafer.
 5. The method of claim 3 further comprising: scheduling automatic delivery of the reticle pod to the enclosure; and assigning a respective priority to each operation of exposing a respective additional reticle to the second gas, wherein the scheduling step schedules each exposing operation based on the respective priority of that respective exposure.
 6. The method of claim 5, wherein the respective priority of exposing a given additional reticle is based on at least one factor from the group consisting of an amount of time until a wafer is to be processed using the given additional reticle, a forecast of wafers to be processed within a predetermined period of time, and an amount of time since the given additional reticle was most recently exposed to the second gas.
 7. The method of claim 2 further comprising exposing the additional reticle to the second gas for a first predetermined period of time after the additional reticle is used to process a wafer but before the additional reticle is stored in the at least one receptacle of the stocker.
 8. The method of claim 1 further comprising operating a controller in a first mode in which the controller causes the second gas delivering means to deliver the second gas to the enclosure during one or more discrete periods according to a schedule.
 9. The method of claim 8 further comprising operating a controller in a second mode in which the controller causes the second gas delivering means to deliver the second gas to the enclosure continuously.
 10. The method of claim 1 wherein the first gas is clean dry air.
 11. The method of claim 1 wherein the second gas is selected from the group consisting of nitrogen and an inert gas. 